1. Field of the Invention
The present invention relates to non-crystallising, amine-blocked polyisocyanates, to their preparation and to their use in one-component polyurethane stoving compositions, in particular for coil-coating applications.
2. Description of the Prior Art
There are only a few isocyanate blocking agents that combine good chemical properties, for example, slight thermal yellowing and low deblocking temperatures, and a reasonable cost. Diisopropylamine is such a blocking agent.
The use of diisopropylamine as an isocyanate blocking agent is known. For example, the preparation and use of a polyisocyanate blocked with diisopropylamine and based on trimerized isophorone diisocyanate is described in EP-96 210.
The use of a mixture of diisopropylamine and malonic ester is disclosed in EP-A 600 314. However, a disadvantage of diisopropylamine is also pointed out, i.e., that with certain polyisocyanates it forms blocked polyisocyanates that crystallise from solution and, thus, are not storage stable.
A technically and economically important polyisocyanate is the polyisocyanate containing isocyanurate groups that is based on 1,6-diisocyanatohexane (trimerized HDI). This polyisocyanate is known to impart particularly good viscoplastic behavior to the polyurethane coating compositions. It makes the coatings pliable and scratch resistant.
Coil coatings and can coatings require particularly good flexibility since they have to participate in the subsequent deformation of the coated metal sheets without being damaged.
In EP-A-600 314 it is pointed out that trimerized HDI cannot be blocked with diisopropylamine to form a crosslinking agent for dissolved one-component polyurethane stoving compositions, which are stable in storage, since these blocked polyisocyanates crystallise out of solution.
An object of the present invention is to provide a polyisocyanate crosslinking agent for one-component polyurethane stoving lacquers, in particular for the coil-coating applications, which is blocked with diisopropylamine, is stable in storage, does not crystallise from solution and shows slight thermal yellowing.
This object can be achieved with the blocked polyisocyanates described hereinafter.